Paper manufacturers typically supply paper to converters in the form of large rolls wound on cores, and the converters finish the paper into newsprint, magazine stock, and other products which are then sold to printers for printing newspapers, magazines, and the like. The gravure and offset printing presses in common use today are continually being improved to increase their speed as well as their width, in a constant effort to increase throughput and reduce the frequency of roll changes. To meet the demands of these faster and wider presses, paper manufacturers are producing larger and wider rolls of paper.
In general, the industry prefers paper to be wound on paper cores as opposed to cores made of other materials such as steel. Accordingly, paper cores are being subjected to greater and greater loads as paper rolls are increased in size to meet the demands of faster and wider presses. These greater loads can lead to failure of the paper core if it is not properly designed and manufactured.
A paper roll is typically supported on a winder or within a press by a pair of slip-fit or expandable mandrels which are inserted into each end of the paper core and engage the inner surfaces of the core. The mandrels are rotatable to permit the paper roll to rotate as required during a winding or unwinding operation. As the paper roll is rotated, a dynamic or cyclic load is imposed on any given region of the paper core, alternating between compression when the region is on top of the core and tension when the core is rotated to bring the region toward the bottom of the core. The heavy cyclic loading of the core can lead to delamination failure of the core, either by ply bond separation or by adhesive failure. The general term for such failures is dynamic strength failure.
Efforts have been made to develop testing devices for determining the dynamic strength of paper cores. For example, U.S. Pat. No. 4,819,488 issued to Morel discloses an apparatus for testing the resistance to cleavage of cardboard tubes. The device includes a rotatably mounted mandrel which has four narrow keying jaws equally spaced about its outer circumference, the jaws being outwardly expandable by turning an adjustment screw. A sample tube is fitted within a metal sleeve having an inside diameter just slightly larger than the outside diameter of the tube, the metal sleeve being intended to simulate the roll of paper wound around the tube. The assembly of the sleeve and sample tube are placed over the mandrel and the adjustment screw is operated to cause expansion of the jaws to engage the inner surface of the tube so as to compress the wall of the tube between the jaws and the metal sleeve. A drive belt is looped about the outer surface of the metal sleeve. The drive belt is stretched between the sleeve and a drive pulley mounted on a drive shaft which is spaced from and parallel to the rotation axis of the mandrel and is rotatably supported by a pivotal frame. The drive pulley is rotatably driven by a motor. Thus, the metal sleeve is rotatably driven by the drive belt and motor, and the sample tube is in turn rotatably driven by the frictional engagement of the metal sleeve with the outer surface of the tube. A pneumatic jack is connected to the pivotal frame and is operable to cause pivotal motion of the frame so as to change the distance between the rotation axis of the mandrel and the axis of rotation of the drive shaft, thereby changing the tension in the drive belt. Thus, the load is imposed on the tube by the inner surface of the metal sleeve, which transmits the load imposed on it by the drive belt. The variable tensioning of the belt is intended to simulate varying weight of a paper roll wound on the tube.
The testing apparatus disclosed in the Morel patent does not accurately simulate the type of loading experienced by a paper core in actual use. More specifically, it can be shown that variation in fit between the outer surface of the test sample and the inner surface of the metal sleeve affects the test results. Since manufacturing tolerances for paper cores can lead to significant variations in outer diameters, this sensitivity to outer diameter represents a significant problem. Furthermore, paper cores are made in a substantial number of different nominal outer diameters, and therefore the Morel apparatus requires that many sleeves of different inside diameters be kept on hand in order to match the test sample with a sleeve of the proper inside diameter.
Additionally, the metal sleeves are quite rigid, and it can be shown that this rigidity adds support to the test sample, thus altering the test results and making detection of defects more difficult. The Morel device also does not include any provision for accurately measuring and controlling the load imposed on the test sample by the variable-tension belt. Moreover, driving the sample by frictional engagement with the inner surface of the sleeve is not representative of the manner in which a paper core is driven in actual use. Yet another problem with the Morel device is that the mandrel differs greatly from the mandrels used in the industry. The keying jaws of the Morel mandrel are quite narrow, the four jaws combined contacting only about 35 percent of the circumference of the test sample. Thus, expansion of the jaws can destroy the paper in the areas where the jaws contact the inner surface of the sample. Further, the load on the test sample causes concentrated compressive forces on the wall of the sample where the jaws contact the inner surface. Both of these mechanisms can significantly alter the test results, or even cause failure of the sample before the test even begins.